This invention generally relates to an improved process of phosphate ore beneficiation by froth flotation. More particularly, this invention relates to such a process wherein phosphate ores are processed by froth flotation using as a collector a fatty acid in combination with a surface active agent.
Froth flotation is the principal means by which phosphate ores, such as pebble phosphate and apatite, and a host of other ores are concentrated. Its chief advantage lies in the fact that it is a relatively efficient process operating at substantially lower costs than many other processes capable of concentrating the ores.
Flotation is a process for separating finely ground valuable minerals from their associated gangue, or waste, or for separating valuable components one from another. In froth flotation, frothing occurs by introducing air into a pulp of finely divided ore and water containing a frothing agent. Minerals that have a special affinity for air bubbles rise to the surface in the froth and are separated from those wetted by the water. The particles to be separated by froth flotation must be of a size that can be readily levitated by the air bubbles.
Agents called collectors are used in conjunction with flotation to promote recovery of the desired material. The agent chosen must be capable of selectively coating the desired material in spite of the presence of many other mineral species. Current theory states that the flotation separation of one mineral species from another depends upon the relative wettability of surfaces. Typically, the surface free energy is purportedly lowered by the adsorbtion of heteropolar surface-active agents. The hydrophobic coating thus provided acts in this explanation as a bridge so that the particle may be attached to an air bubble. The practice of this invention is not, however, limited by this or other theories of flotation.
Typically, phosphate ore containing 15-35% BPL[bone phosphate of lime, Ca.sub.3 (PO.sub.4).sub.2 ] is concentrated in very large tonnages from deposits such as the Florida pebble phosphate deposits. The ore slurry from strip mining is sized at about 1 millimeter and the coarser fraction, after scrubbing to break up mud balls, is a finished product. The minus 1 mm fraction is further sized at 35 and 200 mesh. The minus 200 mesh slime is discarded. From the sizing operation, the +35 mesh material in thick slurry is treated with fatty acid, fuel oil and caustic, ammonia or other alkaline material and the resulting agglomerates are separated on shaking tables, spirals, spray belts or flotation. The 35-200 mesh fraction is conditioned with the same type of reagents and floated by conventional froth flotation routes. Not all the silica gangue is rejected by the fatty acid flotation so the concentrate is blunged with acid to remove collector coatings, deslimed, washed free of reagents and subjected to an amine flotation with fuel oil at pH 7-8. This latter flotation, sometimes called "cleaning", removes additional silica and raises the final concentrate grade to 70-80% BPL.
The disposal of phosphate slimes generated in the beneficiation of phosphate ore, traditionally accomplished through the employment of slime ponds, has come under increased opposition from environmental grops as well as those interested in a more rapid reclamation of the water and land involved. To help resolve these objections, organic polymers have been frequently employed to flocculate and settle the slimes more rapidly. This particular solution, however, has brought with it related problems of a different nature. Specifically, the clear overflow water, which is recycled through the process, can contain residual organic polymeric flocculants which cause severe depression of the fatty acid collectors when employed in the beneficiation process. To overcome this effect, and thereby maintain acceptable recovery values, as much as double the normal amount of fatty acid collector may be required. It has been estimated that the total consumption of fatty acid products used by the Florida phosphate industry alone exceeds 100 million pounds per year. Doubling that amount not only would entail additional expenses but would divert large quantities of fatty acids from nutritional and other uses to which they might otherwise have been employed.
Thus, there exists the need for a process employing a phosphate ore collector whose selectivity is not depressed by residual amounts of organic polymeric flocculants contained in the recycled water. Additionally, in light of the requirements for fatty acids in nutritional and other areas and the high quantities of phosphate minerals being processed by froth flotation, a process wherein such a collector which does not require increased amounts of fatty acids would be particularly desirable. Accordingly, the provision for an improved process for beneficiating phosphate ore in the presence of residual polymers would fulfill a long felt need and constitute a significant advance in the art.